Technical Field
Embodiments described herein relate to methods for manufacturing semiconductor wafers, and to semiconductor devices having a low concentration of interstitial oxygen. Further embodiments relates to methods for manufacturing semiconductor wafers having a resistivity in a given range.
Background Art
Semiconductor devices are processed on semiconductor wafers which are thin plates of cut large semiconductor crystals, which are referred to as ingots. There are basically two different methods to produce semiconductor ingots: methods based on Czochralski process (CZ process) and methods based on the float-zone process (FZ process). The FZ process allows manufacturing of ingots with a very low concentration of light impurities. However, the size of the ingots which can be produced by FZ processes is limited to about 200 mm in diameter. Furthermore, FZ processes are more expensive than CZ processes. Different to FZ processes, ingots having a large diameter of 300 mm (12 inch) or larger can be manufactured by CZ processes.
For certain devices such as IGBTs a low concentration of interstitial oxygen and a high intrinsic resistivity in a given range are desirable. FZ processes allow the manufacturing of ingots having a sufficiently low concentration of interstitial oxygen at high cost. The concentration of oxygen in CZ crystals is inherently higher than for FZ crystals since a quartz crucible is in direct contact with the hot melt which adds oxygen to the melt. To adjust the resistivity of CZ ingots, dopants can be added to the molten semiconductor material. However, due the segregation effect, the dopants are enriched in the molten semiconductor material during formation of the ingot. The manufactured CZ ingot thus has a doping gradient in its longitudinal direction of 50% or more. Such a variation is too large for many semiconductor devices, particularly power devices, so that a large portion of the ingots cannot be used for the intended purpose. This further increases the manufacturing costs.
In view of the above, there is a need for improvement.